1. Field of the Invention
This invention relates to an improvement in a signal reproduction apparatus, by which electric signals are reproduced from a record disk.
2. Prior Arts
A signal reproduction apparatus by use of a record disk, on which a large number of circular grooves are formed has been disclosed in, for example, U.S. Pat. Nos. 3,652,809 for Dickopp et al., 4,037,253 for Nagaoka, and 4,145,718 for Dholakia et al. The record disk is provided with a large number of guide grooves either in a spiral form or a concentric circular form. The bottom and/or the walls of the guide grooves are modulated by means of signals, such as video signals including sound signals, so as to mechanically store the signals in the form of engraved patterns. When reproducing the recorded signals, the record disk groove is traced thereby detecting the displacement of a playback stylus caused by the engraved patterns, or detecting changes of an electrostatic capacity between an electrode provided on the stylus and an engraved patterned conductor provided under a smooth plastic layer over the groove patterns. In such record disk reproducing apparatuses, there arise problems due to dust attached to the surface of the record disk. The dust in the air is likely to absorb moisture and/or oil, and become strongly attached to the record disk surface. It is liable to obstruct the tracking motion of the playback stylus and cause jumping of the playback stylus. In the currently used video disks, the pitch interval of the recording tracks is usually in the range of only several microns, and therefore one jumping motion of the playback stylus, even if a slight one, makes for considerable reproduction failure of the signals recorded in the several recording tracks.
In addition, the foreign matters consisting of the dust and a lubricant material coating the record disk surface often occupy the engraved grooves, and therefore, so-called locked grooves are formed. Once the locked groove is formed, the playback stylus repeatedly traces the same recording track and the reproduced video image becomes a still image.
Conventionally, in order to avoid such troubles, it has been proposed to provide a pre-play stylus which scans on the surface of the recording disk in advance of the playback stylus, so as to remove the foreign matters occupying the engraved grooves by the pre-play stylus. In a conventional apparatus, a pre-play stylus (hereinafter referred to as a shaving means) made of a hard material such as diamond or sapphire has several teeth for engaging with the grooves formed on the surface of a recording disk.
The present inventors studied the tracking motion of such a conventional shaving means and found the following problems. The conventional shaving means removes the foreign matters attached to the recording disk surface, but it also shaves to some extent the walls and bottoms of the grooves thereby spoiling the quality of the recorded signal pattern. It is naturally true that all of the recorded signal patterns are not necessarily shaved at once, but the signal to raise ratio decreases earlier in the midway of the life time of the recording disk as compared to the case when the reproduction is made only by a playback stylus without a shaving means. It was also revealed after the life test of recording disks that the recorded signals were no more accurately reproduced without drop-out of signals at an earlier stage of the life, when the signal reproduction is carried out by a playback stylus together with a shaving means with several teeth.
Further, the present inventors studied the tracking motion by use of a shaving means having a more gentle angle at the tip of the teeth than that of the grooves. Such a shaving means is employed in order to decrese the contact area of the teeth with the walls of the grooves. But it was revealed that the teeth were soon rubbed by the walls of the engraved grooves resulting in a teeth shape having the same tip angle as that of the grooves, and that the worn teeth scrubbed the engraved walls of the grooves.
On the other hand, a conventional video disk, depending on the type, is provided with a thin coating layer of a lubricant on the surface thereof in order to decrease the abrasion of the playback stylus and the record disk. It was revealed in the test of applying a shaving means with teeth for such a record disk that the shaving means scrubbed the lubricating coating layer and that the playback stylus and the record disk were remarkably abraded. It is easily understood that the abrasion of the recorded signal patterns in the grooves and the scrubbing of the lubricating coating layer are accelerated when the number of the teeth at the shaving means becomes large, since the grooves are scrubbed as many times as the number of the teeth.